Communication systems take many forms. Reference may be made to a book by Robert C. Dixon entitled Spread Spectrum Systems, John Wiley & Sons, New York, 1984, which describes many aspects of communication systems. In general, the purpose of a communication system is to transmit information-bearing signals from a source, located at one point, to a user destination, located at another point some distance away. A communication system generally consists of three basic components: transmitter, channel, and receiver. The transmitter has the function of processing the message signal into a form suitable for transmission over the channel. This processing of the message signal is referred to as modulation. The function of the channel is to provide a physical connection between the transmitter output and the receiver input. The function of the receiver is to process the received signal so as to produce an estimate of the original message signal. This processing of the received signal is referred to as demodulation.
Two types of two-way communication channels exist, namely, point-to-point channels and point-to-multipoint channels. Examples of point-to-point channels include wirelines (e.g., local telephone transmission), microwave links, and optical fibers. In contrast, point-to-multipoint channels provide a capability where many receiving stations may be reached simultaneously from a single transmitter (e.g., cellular radio telephone communication systems). These point-to-multipoint systems are also termed Multiple Address Systems (MAS).
Analog and digital transmission methods are used to transmit a message signal over a communication channel. The use of digital methods offers several operational advantages over analog methods, including but not limited to: increased immunity to channel noise and interference, flexible operation of the system, common format for the transmission of different kinds of message signals, improved security of communication through the use of encryption, and increased capacity.
These advantages are attained at the cost of increased system complexity. However, through the use of very large-scale integration (VLSI) technology, a cost-effective way of building the hardware has been developed.
To transmit a message signal (either analog or digital) over a band-pass communication channel, the message signal must be manipulated into a form suitable for efficient transmission over the channel. Modification of the message signal is achieved by means of a process termed modulation. This process involves varying some parameter of a carrier wave in accordance with the message signal in such a way that the spectrum of the modulated wave matches the assigned channel bandwidth. Correspondingly, the receiver is required to recreate the original message signal from a degraded version of the transmitted signal after propagation through the channel. The re-creation is accomplished by using a process known as demodulation, which is the inverse of the modulation process used in the transmitter.
In addition to providing efficient transmission, there are other reasons for performing modulation. In particular, the use of modulation permits multiplexing, that is, the simultaneous transmission of signals from several message sources over a common channel. Also, modulation may be used to convert the message signal into a form less susceptible to noise and interference.
For multiplexed communication systems, the system typically consists of many remote units (i.e., subscriber units) which require active service over a communication channel for short or discrete intervals of time rather than continuous sevice on a communication channel at all times. Therefore, communication systems have been designed to incorporate the characteristic of communicating with many remote units for brief intervals of time on the same communication channel. These systems are termed multiple access communication systems.
One type of multiple access communication system is a frequency division multiple access (FDMA) system. In a FDMA system, the communication channel is divided into several narrow frequency bands. Individual communication channel links are established between two communication units within one of these narrow frequency bands. These communication links are maintained for discrete amounts of time while the two communication units transmit and receive signals. During particular communication links between the two communication units, the communication system does not allow other communication units access to the narrow frequency band within the communication channel which is being utilized by the communication units in the particular communication link.
Another type of multiple access communication system is a time division multiple access (TDMA) system. In a TDMA system, the communication channel is divided into time slices of a time frame to allow communication links between two communication units to exist in the same communication channel simultaneously, but at different time slices. This is accomplished by assigning particular time slices of a time frame to a particular communication link and other time slices to other communication links. During these particular communication links between the two communication units, the communication system does not allow other communication units access to the time slice of the time frame within the communication channel which is being utilized by the communication units in the particular communication link.
Further, another type of multiple access communication system is a spread spectrum system. In a spread spectrum system, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band within the communication channel. The frequency band is much wider than the minimum bandwidth required to transmit the information being sent. A voice signal, for example, can be sent with amplitude modulation (AM) in a bandwidth only twice that of the information itself. Other forms of modulation, such as low deviation frequency modulation (FM) or single sideband AM, also permit information to be transmitted in a bandwidth comparable to the bandwidth of the information itself. However, in a spread spectrum system, the modulation of a signal to be transmitted often includes taking a baseband signal (e.g., a voice channel) with a bandwidth of only a few kilohertz and distributing the signal to be transmitted over a frequency band that may be many megahertz wide. This is accomplished by modulating the signal to be transmitted with the information to be sent and with a wideband encoding signal.
Unlike FDMA and TDMA systems, in spread spectrum systems, a signal may be transmitted in a channel in which the noise power is higher than the signal power. The modulation and demodulation of the message signal using spread spectrum techniques provides a signal-to-noise gain which enables the recovery of the message signal from a noisy communication channel. The greater the signal-to-noise ratio for a given system equates to: (1) the smaller the bandwidth required to transmit a message signal with a low rate of error, or (2) the lower the average transmitted power required to transmit a message signal with a low rate of error over a given bandwidth.
Three general types of spread spectrum communication techniques exist, including:
the modulation of a carrier by a digital code sequence whose bit rate is much higher than the information signal bandwidth. Such systems are referred to as "direct sequence" modulated systems.
carrier frequency shifting in discrete increments in a pattern dictated by a code sequence. These systems are called "frequency hoppers". The transmitter jumps from frequency to frequency within some predetermined set; the order of frequency usage is determined by a code sequence. Similarly "time hopping" and "time-frequency hopping" have times of transmission which are regulated by a code sequence.
pulse-FM or "chirp" modulation in which a carrier is swept over a wide band during a given pulse interval.
Information (i.e., the message signal) can be embedded in the spectrum signal by several methods. One method is to add the information to the spreading code before it is used for spreading modulation. This technique can be used in direct sequence and frequency hopping systems. It will be noted that the information being sent must be in a digital form prior to adding it to the spreading code because the combination of the spreading code, typically a binary code, involves module-2 addition. Alternatively, the information or message signal may be used to modulate a carrier before spreading it.
Thus, a spread spectrum system must have two properties: (1) the transmitted bandwidth should be much greater than the bandwidth or rate of the information being sent, and (2) some function other than the information being sent is employed to determine the resulting modulated channel bandwidth.
The essence of the spread spectrum communication involves the art of expanding the bandwidth of a signal, transmitting the expanded signal, and recovering the desired signal by remapping the received spread spectrum into the original information bandwidth. Furthermore, in the process of carrying out this series of bandwidth trades, the purpose of spread spectrum techniques is to allow the system to deliver error-free information in a noisy signal environment.
Spread spectrum communication systems can be multiple access systems like FDMA and TDMA communication systems. One type of multiple access spread spectrum system is a code division multiple access (CDMA) system. In a CDMA system, communication between two communication units is accomplished by spreading each transmitted signal over the frequency band of the communication channel with a unique user spreading code. As a result, transmitted signals are in the same frequency band of the communication channel and are separated only by unique user spreading codes. Particular transmitted signals are retrieved from the communication channel by despreading a signal representative of the sum of signals in the communication channel with a user spreading code related to the particular transmitted signal which is to be retrieved from the communication channel. A CDMA system may use direct sequence or frequency hopping spreading techniques.
Initial synchronization of signals between two communication sites which are communicating with each other in a spread spectrum communication system is an important aspect of the process of transmitting signals between the two communication sites. Synchronization of the two communication sites is necessary to allow the despreading of the received signals by a spreading code which is synchronized between the two communication sites so that the originally transmitted signal can be recovered from the received signal. Synchronization is achieved when the received signal is accurately timed in both its spreading code pattern position and its rate of chip generation with respect to the receiving communication site's spreading code.
One of the problems associated with synchronization is that the techniques used to synchronize two signals are relatively expensive to implement. In communication systems having sophisticated and relatively expensive central communication sites which serve a plurality of relatively inexpensive remote communication sites, it is desirable to reduce the cost of synchronization systems in the remote communication sites while not increasing the cost of the central communication sites. The present invention can be implemented in such central/remote communication site systems to reduce the cost of the remote communication site synchronization hardware/software while nominally increasing the synchronization hardware/software in the central communication sites.
In spread spectrum systems, with respect to synchronization, two general areas of uncertainty of the signal exist which must be resolved before a received spread spectrum signal can be recovered. These areas of uncertainty are spreading code phase and carrier frequency. In addition, spreading code clock rate can be a source of synchronization uncertainty. Most of this uncertainty may be eliminated by utilizing accurate frequency sources in both communication sites which are communicating with each other. However, some uncertainty cannot be eliminated by the use of accurate frequency sources. Doppler-related frequency errors typically cannot be predicted and will affect the carrier frequency. The amount of Dopper-related frequency uncertainty present in a received signal is a function of the relative velocity of the receiver which received the signal with respect to the transmitter which transmitted the signal, as well as the frequency (or frequency range) at which the signal was transmitted. Further, if at least one of the two communication sites in a communication link is mobile communication site, then a relative spreading code phase change will occur with each change in relative position of the mobile communication site with respect to the other communication site in the communication link. Furthermore, fixed-position communication sites can experience variations in spreading code phase and carrier frequency due to signal propagation-path-length changes in the communication channel.
One of the simplest of all synchronization techniques involves using a sliding correlator. In the sliding correlator, a spreading code generator operates at a rate different from the rate at which a spreading code generator associated with a transmitter which transmitted the signal to be correlated operates. The effect is that the two spreading code sequences slip in phase with respect to each other, and if viewed simultaneously, the spreading codes would seem to slide past each other until the point of coincidence is reached.
More particularly, a sliding correlator receives a spread spectrum signal which is a function of a particular spreading code and generates a signal locally which is a function of a locally-generated spreading code which is substantially similar to the particular spreading code. Subsequently, the sliding correlator compares the received signal with the locally-generated signal. If the two signals are not determined to be aligned, then the sliding correlator phase shifts the local signal with respect to the received signal and loops back to compare the phase shifted local signal with the received signal. This process continues until the sliding correlator determines that the two signals are aligned at which point the total phase shift of the local signal is stored by the sliding correlator for subsequent use. The total phase shift and the locally-generated spreading code are used to despread subsequently received spread spectrum signals which have been spread with spreading codes which are substantially similar to the locally generated spreading code, but phase-shifted.
The primary advantage of the sliding correlator is its simplicity. However, the primary disadvantage in the use of sliding correlators in spread spectrum communication systems has been that a relatively large amount of uncertainty of the relative spreading code phase between transmitted and received spread spectrum signals has existed. Thus, it has been necessary to compare most of the possible spreading code phase positions of a locally generated signal to a received signal. This extensive comparison has been impractical for real-time operations such as radiotelephone communication. Therefore, it has been necessary to find ways to decrease the time of synchronization while using a sliding correlator. One technique is to limit the range of phase positions which must be compared before synchronization can be accomplished.
One way to limit the range of phase positions which must be compared is to use preamble synchronization sequence. Preamble synchronization sequences are short spreading codes (e.g., 100 to 10,000 bits). The time of synchronization of a received signal is directly related to the length of the spreading code. Because the spreading codes are short, the time of synchronization for a received signal is short. After synchronization, communication between communication sites is continued with a longer spreading code (e.g., 10.sup.20 bits or more in length). Unfortunately, the use of preamble synchronization sequences having relatively short spreading codes has a weakness. The weakness is that these short spreading codes tend to be more vulnerable to false correlations due to noise in the received signal because there are a relatively small number of maximal length spreading codes for these short spreading codes. Thus, the use of short spreading codes, in a large communication system, would require communication sites to share the same maximal length spreading codes, which leads to possible false correlation with a communication site to which the signals were intended to be sent. Therefore, to eliminate false correlations the use of long spreading codes (e.g., on the order of 10.sup.20 bits or more) is preferable.
Another closely-related way to limit the range of phase positions which must be compared is to use easily acquirable or synchronizable spreading codes. An example of an easily acquirable spreading code is a JPL component code. JPL component codes are made up to a plurality of shorter length maximal codes, each of a different length. Because JPL component codes consist of more than one maximal length code, they have one more autocorrelation point than component in the code (e.g., if a JPL code is made up of a 2.sup.5 -1 maximal code, a 2.sup.4 -1 maximal code and a 2.sup.3 -1 maximal code, then there are four autocorrelation points associated with the JPL code). In contrast, a single maximal code having a similar length as the JPL code would have a single autocorrelation point. Moreover, all but one of these autocorrelation points are associated only (and separately) with the individual codes making up the composite JPL code. The highest correlation point corresponds to total composite spreading code synchronization.
Synchronization, by using JPL component codes, is accomplished by first cross-correlating one of the component codes with the composite code. Once this component code reaches the point of synchronization with its mate, which is embedded in the composite code, a partial correlation occurs. The partial correlation is then the signal for the second component code cross-correlation to be initiated, which causes the partial correlation level to be increased. This process continues until all of the component codes making up the overall composite code are individually synchronized with their counterparts in the received signal. When all are individually synchronized, the correlation is the same as if the process had simply synchronized the composite code. The advantage of this technique is that it provides for rapid synchronization acquisition without the use of a preamble or anything other than the composite code itself. When the component codes are, for example, 200, 500 and 100 bits in length, then separate search processes over these individual lengths (a total of 1700 bits) can be accomplished much more rapidly than a search of the composite 10.sup.8 bits. However, this advantage in synchronization time is paid for by a decrease in the signal-to-noise ratio in the correlator output when all code components are not synchronized (i.e., susceptible to noise and interference).
The present invention overcomes the disadvantage of a sliding correlator by determining a maximum range of phase positions of a locally-generated signal that a sliding correlator must compare to a received signal. In addition, the present invention does not exhibit the vulnerability of the spreading code to false correlation that the use of synchronization preambles exhibits or the susceptibility to noise and interference that the use of JPL component code exhibits.